1. Field of the Invention
The present invention relates to an optical sampling method, system and program for monitoring repeatedly entered high speed signal lights, especially signal lights that are modulated at a speed that cannot be monitored by an ordinary photo-detector element or electric circuit.
2. Description of the Related Art
Conventional optical sampling systems have included a technique for monitoring low power and high speed repeating optical signals by monitoring linear correlation between signal lights and locally generated optical pulses. See Japanese Patent Application Laid Open No. 9-162808 (1997).
As shown in FIG. 2, in this conventional optical sampling system, the locally generated optical pulses from a sampling optical pulse generation unit 1 and the signal lights are injected into an optical hybrid 8, a plurality of output lights from this optical hybrid 8 are received by balanced optical receivers 5a and 5b, and output signals of these balanced optical receivers 5a and 5b are squared by square circuits 6a and 6b and then added together by an adder 7, so as to obtain an output proportional to an intensity of the signal lights at a time when the locally generated optical pulses and the signal lights overlap with each other. In this way, it is possible to realize an optical sampling operation with a time resolution equal to a pulse width of the locally generated optical pulses, without depending on a processing speed of electric circuits.
In general, the optical signals often receive intensity modulation, as well as frequency modulation. For example, when modulating using the ON/OFF control of the injection current of a semiconductor laser, the intensity is of course changed, but at the same time the frequency of the generated laser beam is also changed. Also, in the case where a modulated laser beam is transmitted through an optical fiber, the frequency of the optical signals after the transmission will be modulated due to the nonlinear optical effect of the optical fiber, which is called self phase modulation.
Thus, the conventional optical sampling system can monitor the intensity change of the signal lights, but cannot obtain any information regarding how the frequency has been changed.